Conventional planar magnetron sputtering of ceramic materials (such as for sputtering of ITO, AZO, TiO2, Nb2O5, and/or the like on a glass substrate of a mirror reflective element) uses ceramic tiles that are bonded (such as by indium solder, elastomer, nanofoils and/or the like) to a metal backing plate (such as Copper, Copper alloys, Molybdenum and/or the like). Typically, the backing plate forms a support structure for the tiles, provides a path for the electrical power (RF, AC, DC, pulsed) and acts as a thermal sink so that heat is removed from the tiles during the sputtering process.
For example, and with reference to FIG. 1, a conventional magnetron sputtering assembly 10 for sputter coating a substrate 12 (such as a glass substrate of a mirror reflective element) with a coating includes a backing plate 14 with a target 16 soldered to the backing plate 14 via a layer of solder 18. The backing plate 14 is on a magnet assembly 20, and an anode 22 is provided at the target. The magnet assembly 20 is provided at the backing plate and at an opposite side of the target from the substrate 12 to generate a magnetic field for focusing or containing the sputtering at the target. The conductive anode 22 is disposed between the substrate and the target, and a sputtering current is applied to sputter the target in a known manner.
There are several disadvantages with such known approaches. For example, during the sputtering process, the tiles may tend to heat up. There can be a steep thermal gradient from the top of the tile to the area where the tile is bonded to the backing plate, and such a gradient in conjunction with the differences in the coefficients of thermal expansion of the different materials can cause the tiles to crack or delaminate from the backing plate. Cracking can exacerbate the tendency of the target to arc, which typically results in process instability and particulate ejection from the target surface causing damage in the deposited film. Large cracks in the tiles can cause the bonding material and backing plate material to sputter and contaminate the depositing film.
Another disadvantage of such known approaches is the cost associated with such approaches, since the bonding process can be expensive and it typically involves many steps. Tiles are typically sputter coated with various layers to prevent diffusion of the indium solder through the tile and adhesion layers to promote adhesion to the backing plate. Flux is applied so that the molten indium wets the bonding surfaces. Small gaps (such as about 0.015 inches) need to be set between the tiles during the bonding process to allow for expansion. In order to prevent molten solder from filling the gap by capillary action, Kapton tape is typically applied to the backing plate in the area of the bond gap. To ensure a uniform solder thickness, thin metallic wires or spacers (such as about 0.3 mm thick) are typically used to set the tile distance from the backing plate.
Although the bonding process is typically expensive, it may be carried out successfully on a commercial basis. However, there is another disadvantage with the conventional processes that is not well understood. The ceramic materials tend to form nodules (such as described in M. Schlott et al., P-31: Nodule Formation on Indium-Oxide Tin-Oxide Sputtering Targets, SID Digest, 1997, which is hereby incorporated herein by reference in its entirety) on the surface of the target during sputtering. The surface density of nodules increases with time and is a source of target arcing, particulate formation, film contamination, sputtering rate decrease and overall process instability.